The invention relates to a rotary-wing aircraft rotor with constant velocity drive, in particular for a convertible aircraft supporting two generally fixed wings and at least one tilting rotor.
Although the constant velocity drive rotor of the invention can be used as a helicopter rotor, in particular as a tail or anti-torque rotor, a particularly advantageous application of such a constant velocity drive rotor consists in fitting it to convertible aircraft with tilting rotors, particularly of the type known and described in FR 2 791 319, FR 2 791 634 and FR 2 798 359, to which reference may advantageously be made for further details. Briefly, such a convertible aircraft with tilting rotors comprises, as shown schematically in FIG. 1, a fuselage 1, of the aircraft fuselage type, supporting two fixed wings 2, for example high wings, raised with respect to the fuselage 1, each wing 2 itself supporting a power pod 3, housing a power unit driving in rotation a rotor 4, represented schematically by the plane of rotation of the rotor blades, via a transmission (not shown in FIG. 1), a rear reduction gear unit of which is directly driven by the power unit and housed like the latter in the stationary rear part 5 of the power pod 3. The front part 6 of the power pod 3, housing a rotor mast and a rotor hub, as well as a front reduction gear unit driving the rotor mast in rotation, is mounted so as to pivot with the rotor 4, so that it can pivot from an aeroplane configuration, in which the rotor 4 operates as a propeller at the front of an engine pod 5-6 facing into the relative wind, to a helicopter configuration, in which the rotor 4xe2x80x2 operates as a helicopter main lifting rotor at the upper end of the front pivoting part of the pod in the upright position 6xe2x80x2 above the corresponding wing 2, this latter configuration being used for takeoff and landing of the convertible aircraft which, after transition from the helicopter configuration to the aeroplane configuration, is able to move in forward flight like an aircraft. As a variant, the pods 3 may pivot totally with the rotor 4 with respect to the fixed wings 2.
On rotary-wing aircraft rotors, it is known that, since the introduction of the flapping hinge on autogyro and helicopter rotors, tilting the rotor where coning is present, whether this tilting is desired and generated by controlling the cyclic pitch or the unwanted result of the asymmetry between an advancing blade and a retreating blade, causes stresses in the drive plane of the blades which tend to cause the blades to accelerate and decelerate in the course of a revolution of the rotor. These variations in speed are caused by Coriolis forces, and may be illustrated simply by the fact that the trajectory of the blade tips, viewed in a plane perpendicular to the drive axis, is an eccentric ellipse, the angular rate of travel of which is constant and, consequently, the peripheral speed of which varies over a revolution. These accelerations and decelerations of the blades over a revolution of rotation have a disastrous effect on the service life of the rotor components, due to the fact that these variations in speed generate stresses which are all the more substantial because the rigidity of the rotor components is high.
Conversely, it is known that great flexibility along the drag axis of the blades has a highly beneficial effect on the dynamic stresses to which the blades and the components of the rotor hub are subjected, which is why the introduction of the flapping hinge has been accompanied by the introduction of the drag hinge.
These improvements to the original rotary-wing aircraft rotor concepts have led to a rotor fully articulated in pitch, flapping and drag, the main disadvantage of which was to be subject to instability of the ground resonance or air resonance type, which made it necessary to develop and use drag dampers, also known as frequency adapters, or again elastic return drag struts with built-in damping. On helicopter rotors, these drag dampers are arranged in the plane of rotation of the rotor, between the blades and the hub of the rotor in a conventional configuration, or between adjacent blades of the rotor in the inter-blade configuration. In both cases, the presence of the drag dampers increases the aerodynamic drag of the rotor, in particular at the hub and where the hub is connected to the blades, which reduces the overall performance of the helicopter.
On a convertible aircraft of the tilting rotor type presented above, in which the speed of travel in the aeroplane mode is far higher than that of the helicopter, and on which drag dampers, mounted as on a helicopter rotor, would be head on to the wind, this reduction in performance would be far more appreciable, which is why designers of convertible aircraft of this type have endeavoured, for the design of the rotors, to retain hubs which are extremely rigid in drag (known as stiff-in-plane rotors), with no drag dampers, the natural drag frequency of which is greater than the nominal frequency of rotation of the rotor, which eliminates any risk of instability in drag, even in the absence of drag dampers.
However, it is known that rotors which are rigid in drag have the major disadvantage of generating very high stresses when the rotors are tilted. On convertible aircraft, the importance attached to producing rotors of high aerodynamic efficiency, and therefore with no drag dampers, has led to the development of hubs which are not sensitive to Coriolis forces. A particular feature of these hubs, which include hubs with a universal joint drive, is that tilting of the rotor is accompanied by tilting of the drive axis of the latter. Because of this, the rotor drive axis is always perpendicular to the rotor plane, and the trajectory described by the blade always remains a circle in a plane perpendicular to the drive axis of the rotor. This type of drive has been used on prototype convertible aircraft, particularly the XV15 aircraft.
However, a known particular feature of universal joints is that they are not of the constant velocity type, which manifests itself by the fact that the output speed of these joints is not always equal to the input speed. This speed distortion occurs when the drive and output axes are not co-linear, i.e. in the application considered to driving a rotor in rotation, when cyclic flapping is present. In the simplest configuration of a universal joint, the latter comprises a spider, the joints of which, by one arm of the spider to a driving shaft and by the other arm of the spider to a driven shaft, allow the driven or output shaft to swivel relative to the driving or input shaft. It is known that these speed variations caused by such a universal joint, and transmitted to the driven shaft, correspond to accelerations and decelerations which, over one revolution of rotation of the universal joint, appear twice. The speed of the driven shaft is therefore not constant, but varies at a frequency equal to twice the frequency of rotation of the shafts.
To eliminate these speed variations, which are responsible for very substantial inertial forces, in the case of a rotary-wing aircraft rotor, which affect the hub as a whole and are prejudicial to the durability of the mechanical assemblies constituting the hub or associated with the latter, several constant velocity drive systems have been proposed, particularly so-called Clemens drive links, composed of assemblies of two branches hinged respectively to the driving and driven shafts and connected by a swivel, and also tripod joints, for which transmission of movement is provided by means of balls moving in axial grooves machined in the driving and driven shafts.
These arrangements are used to ensure that the drive point is always situated in a plane bisecting the driving and driven axes. As the distances from this point to the two axes are then identical, the speeds of rotation of the two shafts are strictly equal whatever the angular position of the two shafts, which guarantees that the transmission provides a constant velocity drive.
These two known constant velocity drive systems are not suitable for application to convertible aircraft rotors for the following reasons:
installing Clemens drive links on a convertible aircraft rotor hub very substantially increases the drag of the hub, which reduces its performance and increases operating costs;
tripod joints in particular are not suitable because of the high torque levels encountered on convertible aircraft rotors, which require large diameter and therefore heavy balls to keep the contact surface Hertz pressures at acceptable levels.
In other arrangements, the swivelling and drive functions are kept separate. This is the case in the constant velocity drive system of the V22 tilting rotor convertible aircraft, in which the swivelling function is provided by two halves of a spherical laminated flapping thrust bearing enclosing the hub and connected to the rotor mast. This function absorbs the lift and the coplanar loads due to the aerodynamic and inertial excitation of the rotor. The mast drives the rotor (transmits the torque) by three drive links each connected at one end to the hub and at the other end to the mast.
A variant of this system is proposed in patent U.S. Pat. No. 5,145,321, in which the drive function is provided by substantially parallelepiped-shaped swivel bearings.
A particular feature of these separate means providing the swivelling and drive functions of the hub relative to the rotor mast is that they are kinematically not compatible in the absence of flexibility of the elements connecting the hub to the mast, and constant velocity drive is obtained only by careful tailoring of the rigidity of these connecting elements. Where cyclic flapping of the rotor is present, each drive link mentioned above is subject to dynamic stress at a frequency equal to twice the frequency of rotation of the rotor, the phase depending on the position of this link relative to the hub. For regularly spaced links, in a circumferential direction about the axis of rotation, the phase difference between the dynamic loads on the links is such that the contributions to the dynamic torque cancel each other out, which is a necessary and sufficient condition for constant velocity drive of the hub by the rotor mast. Another major disadvantage of this type of hub in addition to the disadvantage regarding the need for accurate tailoring of drive link rigidity is that the enclosing arrangement of the halves of the flapping thrust bearings make it difficult to inspect the links in particular and the torque transmission system in general, as well as impairing accessibility for maintenance purposes.
In patent U.S. Pat. No. 5,145,321 mentioned above, the vertical shear of a substantially parallelepiped-shaped swivel bearing allows the rotor to pivot about an axis perpendicular to the axis joining the centre of the swivel bearing to the rotor drive axis. Movement of the rotor about a second pivot axis is made possible by the ball joint fitted inside the parallelepiped-shaped bearing. In the same way as for a system where the hub is driven by the mast via links, as presented above, a minimum number of three bearings with closely similar levels of rigidity is required to obtain a constant velocity drive. The flexibility required for correct operation is also directed according to the direction of drive in rotation.
The problem addressed by the invention is to propose a constant velocity drive rotary-wing aircraft rotor, in particular for a convertible aircraft with at least one tilting rotor, the constant velocity drive mechanism of which has the following degrees of freedom:
two degrees of freedom of rotation about two coplanar axes, for pivoting of the hub and therefore of the rotor,
no degrees of freedom in translation, which is equivalent, in terms of loads, to:
the loads, i.e. lift and the coplanar loads, being applied along the two axes considered above and along the axis about which the drive in rotation takes place, and:
the moments being applied about the axis of rotation of the rotor mast only, which corresponds to the drive torque of the hub, the swivelling capability of this mechanism being therefore only partial, since it offers no freedom of rotation of this mechanism about the mast, the rotor of the invention providing a solution to the disadvantages of state-of-the-art rotors of this type, and such as presented above, by being lighter, simpler, more accessible and therefore more economical to produce and maintain and, more generally, in which the functions of swivelling and driving the hub respectively relative to and by the rotor mast do not necessarily have to be kept separate.
To this end, the invention proposes a rotary-wing aircraft rotor with constant velocity drive for a convertible aircraft with at least one tilting rotor, comprising:
a rotor mast, capable of being driven in rotation about its longitudinal axis,
a hub, connected to said mast by a constant velocity drive mechanism and pivoting arrangement, allowing the hub as a whole to pivot about any flapping axis converging with the axis of the mast and perpendicular to said axis of the mast, so that said hub is capable of being driven in constant velocity rotation by said mast, about a geometrical axis of rotation of the hub which may be inclined in any direction about the axis of the mast, and
at least two blades, each linked to said hub by a coupling retaining and hingeing its blade in pitch,
wherein the constant velocity drive mechanism and coupling arrangement comprises:
a first gimbal, driven in rotation by said mast about said axis of the mast, and mounted so as to pivot about a first diametral axis of the mast, which is substantially perpendicular to said axis of the mast, by two first bearings diametrically opposite relative to said axis of the mast,
a second gimbal, also driven in rotation by said mast about said axis of the mast, and mounted so as to pivot about a second diametral axis of said mast, which is substantially perpendicular to said axis of the mast and to said first diametral axis and converging therewith substantially on said axis of the mast, by two second bearings diametrically opposite relative to said axis of the mast,
said first gimbal being in addition hinged to said hub by two first ball joint connections, diametrically opposite relative to said axis of the mast, and each centred substantially in a plane defined by said axis of the mast and second diametral axis,
said second gimbal being in addition hinged to said hub by two second ball joint connections, diametrically opposite relative to said axis of the mast, and each centred substantially in a plane defined by said axis of the mast and said first diametral axis, so that the blades are driven in rotation by two torque transmission trains each comprising said mast, one respectively of the gimbals, the two corresponding ball joint connections and bearings, and said hub, said torque transmission trains having substantially the same torsional rigidity, and one at least of the components of each torque transmission train having flexibility in deformation about the axis of rotation of the hub.
The rotor according to the invention thus comprises means of driving and articulating the hub by and relative to the mast which are based on a universal joint of which the two successive hinges would be combined at the same location between the driving body, the rotor mast, and the driven body, the hub, in such a way that this device has the advantage of simultaneously performing the two functions of swivelling and torque transmission by means of a small number of parts, which makes it relevant in terms of weight, cost and maintenance.
Moreover, in order that these means should be compatible kinematically, it is necessary for the two gimbals to be able to perform small relative angular deflections about this geometrical axis of rotation of the hub. In fact, where the hub is tilted relative to the mast and about an axis not converging with the pivot axes of the gimbals, pivoting of the gimbals in the absence of flexibility between the two torques transmission trains causes rotation of the gimbals in opposite directions about the drive axis of the rotor. Pivoting of one of the gimbals tends to cause the hub to advance, in the direction of rotation of the rotor, whereas pivoting of the other gimbal tends to cause the hub to retreat (rotating in the opposite direction to the direction of rotation of the rotor). To escape from this hyperstatic state, an additional degree of freedom is introduced along the drive axis, and this is obtained by arranging for one at least of the components of each torque transmission train to have flexibility in deformation about the axis of rotation of the hub.
In general, the necessary flexibility about the torque transmission axis may be obtained in the hub, when the latter comprises at least two hub parts made flexible in relative torsion about the axis of rotation of the hub by characteristics of the shape and/or constituent materials of said hub parts, to each of which one respectively of the gimbals is hinged by two corresponding ball joint connections.
Alternately, or in addition, each of the two gimbals may be embodied in a shape and/or of materials providing flexibility in deformation about the axis of rotation of the hub.
At the same time, or alternatively, this flexibility in deformation may be exhibited by the mast, which then comprises two parts made flexible in relative torsion about the axis of the mast, and formed by at least one slot and/or at least one groove and/or one cut-away portion, substantially axial, whether opening to the outside or not, made in the mast, and such that each of the gimbals is pivoted on and driven in rotation by one respectively of said parts of the mast made flexible in torsion.
In all cases, as indicated above, the two torque transmission trains must have substantially the same torsional rigidity, in order for there to be balancing of the dynamic loads at 2xcexa9, where xcexa9 is the frequency of rotation of the rotor, for the drive mechanism according to the invention to provide true constant velocity drive. However, this torsional rigidity of the torque transmission trains must also be compatible with the static stresses (linked to torque) and dynamic stresses (linked to the movements imposed by the kinematics of the device) along the torque transmission axis.
According to a first advantageous embodiment, the mast comprises at least two torsion tubes, having substantially equal rigidity in torsion, and which are integral with each other in rotation about said axis of the mast at one axial end capable of being driven in rotation, each of the two gimbals being mounted so as to pivot on one respectively of the two torsion tubes about one respectively of the two diametral axes. Thus the flexibility required to allow relative rotation of the two gimbals about the drive axis of the hub is provided by the two tubes working in torsion and with loads applied in opposite directions. The substantially equal torsional rigidity of the two tubes allows the dynamic loads at 2xcexa9 to be balanced, so that effectively constant velocity drive is obtained.
In a preferred embodiment, the two torsion tubes are coaxial, arranged one inside the other, and integral in rotation about the axis of the mast at their ends axially on the same side of said mast axis, the inner tube being integral, at its opposite axial end, with a coaxial sleeve on which the corresponding gimbal is mounted so as to pivot about the corresponding diametral axis. Due to their coaxial arrangement one inside the other, the two torsion tubes are made of different materials having a different modulus of elasticity, as the inertia in torsion of the enclosing tube is very probably greater than the inertia of the enclosed tube, so that the enclosing tube or outer tube must be made of a material with a lower modulus of elasticity than that of the material of the enclosed tube or inner tube, to achieve the torsional flexibility of this inner tube.
According to a second advantageous embodiment, the additional degree of freedom along the drive axis, which corresponds to the flexibility required between the two torque transmission trains, is introduced by producing bearings and/or ball joint connections hingeing the gimbals respectively relative to the mast and onto the hub, and, to this end, different embodiments of flexible bearings and/or flexible ball joint connections are possible.
The bearings may be produced on the basis of cylindrical, conical or spherical laminated elements, or a combination of these different forms, so as to allow hingeing of each gimbal relative to the mast with a certain flexibility along the torque transmission axis.
In particular, the bearings pivoting the gimbals relative to the mast are cylindrical and/or conical laminated bearings substantially coaxial about respectively the first and second diametral axes, and preferably with substantially the same radial rigidity. Moreover, these pivot bearings may comprise spherical laminated elements, in which case the latter are centred substantially on one respectively of the diametral axes mentioned previously, and preferably with substantially the same radial rigidity relative to said diametral axes.
The ball joint connections may also be produced from spherical, cylindrical, conical or parallelepiped-shaped laminated elements, or a combination of these forms, so as to enable each gimbal to be hinged relative to the hub, also with a certain flexibility along the torque transmission axis.
In particular, the ball joint connections of the hinges of the gimbals to the hub comprise ball joints associated with substantially coaxial cylindrical and/or conical laminated bearings, providing radial and axial flexibility relative to the corresponding diametral axis, between two rigid members respectively in the inner and outer radial position, a first of which is attached to the corresponding gimbal, and the second of which is attached to the hub.
Advantageously, these ball joint connections of the hinges of the gimbals to the hub comprise laminated ball joints.
Thus, the pivot bearings of the gimbals and the ball joint connections of the gimbals to the hub have sufficient flexibility to allow the gimbals to pivot without excessive strain. In addition, the use of laminated pivot bearings with substantially the same radial rigidity provides balancing of dynamic loads at 2xcexa9, which, as explained above, corresponds to a constant velocity drive.
In a practical manner, the first and second gimbals are driven in rotation about said axis of the mast by respectively the first and a second drive arm, integral in rotation with said mast, and the axes of which are respectively the first and second diametral axes of said mast.
There are thus produced on the mast two drive arms offset by 90xc2x0 and both perpendicular to the axis of the mast, the gimbals being mounted on these so as to pivot and also hinged to the hub by the ball joint connections.
According to an advantageously simple structure, each of the drive arms comprises two end fittings which are axisymmetric about the corresponding diametral axis, diametrically opposite and projecting radially outwardly from said mast, and each retained in one respectively of the two bearings pivoting the corresponding gimbal relative to said mast.
In the variant embodiment, in which the flexibility between the two torque transmission trains is provided by the two torsion tubes mentioned above, the two end fittings of each drive arm advantageously project radially outwardly and are integral with one respectively of the two torsion tubes.
On the other hand, when this flexibility between the two torque transmission trains is provided in the bearings and/or ball joint connections, the two end fittings of each drive arm project radially outwardly and are integral with a central drive barrel fitted around the mast and integral in rotation with the latter about its axis.
In order that the hub may be advantageously rigid in its plane, and that all of the constant velocity drive mechanism and pivoting arrangement is suitably protected, the second member of each combined hinge with ball joint and laminated bearing is advantageously attached to a hub casing which surrounds the two gimbals, their pivot bearings and hinges, and is attached to a hub plate connected to the blades and having a central opening through which said mast runs. The hub plate may then be a plate of known type, of composite material, and in the form of a star with outward-extending arms equal in number to the number of the blades and on each of which are mounted the means for retaining and hingeing a blade in pitch, this arrangement providing good rigidity in drag and a certain flexibility along the flapping axis.
In a preferred embodiment, each laminated bearing for pivoting a gimbal is a conical bearing converging radially outwardly, an outer radial member of which is annular and attached in a cut-away portion of corresponding shape on the corresponding gimbal, and an inner radial member of which is tubular and integral with an end fitting for being driven by said mast.
To improve the rigidity of the rotor in cyclic flapping, the hub may also and advantageously be connected to the mast by at least one elastic assembly for returning the hub to a rest position substantially perpendicular to the axis of the mast.
In a manner known in itself, this elastic return assembly may advantageously comprise at least one half of a central laminated spherical thrust bearing, an outer member of which is connected to the hub and an inner member of which is integral in rotation with the mast. This half of a spherical thrust bearing may be fitted under the hub plate, itself attached below the hub casing enclosing the constant velocity drive mechanism and the pivoting means, but, if in addition the elastic return assembly also comprises an upper half of a central laminated spherical thrust bearing, which covers and encloses said hub casing, a central laminated spherical thrust bearing is then obtained which assists in transmitting to the rotor mast the lift and coplanar loads applied to the rotor.